U.S. Pat. Nos. 1,533,404 and 1,834,063 illustrate blowout preventers which use well pressure to close the well bore. Both patents divert a portion of the well flow within the well bore behind a piston that is connected to a ram which closes the well bore. The cross-sectional area of the piston exposed to the well fluid is larger than the cross-sectional area of the ram shaft exposed to the well fluid and results in a closing force sufficient to close the well bore. Closing of the rams by well pressure requires an external mechanism to open the rams, and is commonly termed "fail-safe" operation because the rams can be closed when the external mechanism fails. U.S. Pat. No. 1,834,063 further describes a separate mechanism for opening and closing the rams by injecting steam, air, or gas on either side of the piston.
U.S. Pat. Nos. 3,036,807 and 4,582,293 describe blowout preventers having balanced rams and ram stems achieved by allowing well pressure behind the ram and behind the ram stem. Pistons connected to the ram stems allow operation of the rams by injecting a hydraulic fluid on either side of the piston. Failure of the hydraulic fluid system makes the blowout preventers inoperable. U.S. Pat. No. 4,582,293 further describes a floating plug in an axial bore of a ram shaft to maintain an essentially constant volume of grease in contact with the outer moving parts of the ram shaft.